1. Field of the Invention
The present invention relates to a cryogenic refrigeration apparatus including a thermal shield, a vacuum vessel and a magnetic shield, which are arranged in that order outwardly of a cryogenic vessel which accommodates a superconducting coil, as well as a refrigerator for cooling the thermal shield.
2. Description of the Related Art
FIG. 1 is a cross-sectional view showing one example of a conventional cryogenic refrigeration apparatus of the type which is used in a magnetic resonance diagnosis system. As illustrated, a liquid helium vessel 1 accommodates a superconducting coil 13 and is charged with liquid helium which serves as a cryogenic cooling medium to maintain the liquid helium vessel 1 at a temperature of 4.2.degree. K. A first thermal shield 2 is disposed outwardly of and in surrounding relationship to the liquid helium vessel 1 which serves as the cryogenic vessel. The first thermal shield 2 serves to reduce the intensity of radiation heat which may enter the liquid helium vessel 1. A second thermal shield 3 is disposed outwardly of and in surrounding relationship to the first thermal shield 2 and serves to reduce the intensity of radiation heat which may enter the first thermal shield 2. A vacuum vessel 4 accommodates the liquid helium vessel 1, the first thermal shield 2 and the second thermal shield 3, and has its interior evacuated for heat insulation. A magnetic shield 14 is disposed outwardly of and in surrounding relationship to the vacuum vessel 4, and prevents magnetic fluxes generated from the superconducting coil 13 from leaking outwardly.
A refrigerator 5 cools the first thermal shield 2 and the second thermal shield 3 through a cooling pipe 5a. The cooling pipe 5a has a first-stage cooling section 6 and a second-stage cooling section 7. A second heat conducting element 8 is connected between the second thermal shield 3 and the first-stage cooling section 6 to conduct heat from the second thermal shield 3 to the first-stage cooling section 6. A first heat conducting element 9 is likewise connected between the second thermal shield 2 and the first-stage cooling section 7 to conduct heat from the first thermal shield 2 to the second-stage cooling section 7. The cooling pipe 5a extends through openings 4a and 3a, which are formed in the vacuum vessel 4 and the thermal shield 3, respectively, and provides communication between the refrigerator 5 and the thermal shields 2 and 3. The cooling pipe 5a, the first-stage cooling section 6, the second-stage cooling section 7 and the first and second heat conducting elements 8 and 9 cooperate to constitute heat conducting means. A bellows 10, which constitutes vibration absorbing means, is disposed between the vacuum vessel 4 and a flange 11 on which the refrigerator 5 is mounted. The bellows 10 holds the vacuum of the vacuum vessel 4 and elastically supports the refrigerator 5 on the vacuum vessel 4 against forces resulting from vacuum pressures so as to absorb the vibration of the refrigerator 5 which is operating. The bellows 10 may be combined with a plurality of springs (not shown) which are disposed between the flange 11 and the vacuum vessel 4. A compressor unit 12 supplies a compressed helium gas to the refrigerator 5, and electrical power to a valve actuating motor (not shown) disposed in the refrigerator 5. An electromagnetic cavity tube 100a defines in its interior an electromagnetic cavity 100 in which a magnetic field is generated by the superconducting coil 13, and is disposed to extend through openings 14b, 4b and 3b which are formed in the magnetic shield 14, the vacuum vessel 4 and the thermal shield 3, respectively. The portion of the electromagnetic cavity tube 100a which extends through the opening 4b of the vacuum vessel 4 is airtightly supported by a flange 4c. These elements constitute access means for providing external access to the magnetic field generated by the superconducting coil 13.
The operation of the conventional cryogenic refrigeration apparatus will be explained below. The amount of heat which enters the liquid helium vessel 1 varies with the change of the thermal shield temperatures. As the temperatures of the thermal shields become lower, the amount of heat which enters the liquid helium vessel 1 becomes smaller, and the consumption of liquid helium in the liquid helium vessel 1 can be reduced. More specifically, during the running of the refrigerator 5, the first-stage cooling section 6 and the second-stage cooling section 7 are cooled to approximately 80.degree. K. and 20.degree. K., respectively. In consequence, the second thermal shield 3 is cooled through the second heat conducting element 8, while the first thermal shield 2 is cooled through the first heat conducting element 9. Accordingly, the consumption of liquid helium is reduced.
However, the conventional cryogenic refrigeration apparatus, which is arranged in the above-described manner, has the problem that the vibration of the refrigerator 5 which is running is transmitted to the thin-walled vacuum vessel 4 and noise occurs due to the vibration of the vacuum vessel 4.